Effect of photoinhibitory treatments on the activity of light-activated enzymes of c(3) and c(4) photosynthetic carbon metabolism

Measurement of photosynthesis in vivo with a leaf disc electrode: correlations between light dependence of steady-state photosynthetic O 2 evolution and chlorophyll a fluorescence transients

The application of a leaf disc electrode to the measurement of the quantum yield of photosynthetic oxygen evolution (in saturating carbon dioxide concentrations) is described. The technique was also used to characterize the light-saturation curves for photosynthesis in leaves of spinach plants grown in relatively low (‘shade’) and high (‘sun’) light intensities. It was found that the flux of a blue excitation light required to elicit oscillations in chlorophyll a fluorescence corresponded, approximately, to the flux of (white) light required to saturate photosynthetic oxygen evolution. The requirement was higher for leaves of sun-grown plants (approximately 800 μmol m -2 s -1 ) than for shade-grown plants (approximately 200 μmol m -2 s -1 ). Brief pretreatment of leaf discs from sun-grown plants with D-mannose did not change the quantum yield, but lowered the light saturated rate and the photon flux density required to saturate photosynthesis. Oscillations in fluorescence could then be observed at lower fluxes of blue excitation light, similar to those required by leaves from shade-grown plants. Photoinhibition of shade-grown plants (exposed to full sunlight for 5 h) reduced quantum yield and increased the light flux required for saturation of photosynthesis. Oscillations in fluorescence, normally observed at low fluxes of blue light in these leaves, could not be detected after photoinhibition. These correlations are interpreted and applications of the techniques described are discussed.

Limitation of CO(2) Assimilation and Regulation of Benson-Calvin Cycle Activity in Barley Leaves in Response to Changes in Irradiance, Photoinhibition, and Recovery

The response of the Benson-Calvin cycle to changes in irradiance and photoinhibition was measured in low-light grown barley (Hordeum vulgare) leaves. Upon the transition from the growth irradiance (280 micromoles per square meter per second) to a high photoinhibitory irradiance (1400 micromoles per square meter per second), the CO(2) assimilation rate of the leaves doubled within minutes but high irradiance rapidly caused a reduction in quantum efficiency. Following exposure to high light the activities of NADP-malate dehydrogenase and fructose-1,6-bisphosphatase obtained near maximum values and the activation state of ribulose-1,5-bisphosphate carboxylase increased. The activity of the latter remained constant throughout the period of photoinhibitory irradiance, but the increase in the activities of fructose-1,6-bisphosphatase and NADP-malate dehydrogenase was transient decreasing once more to much lower values. This suggests that immediately following the transition to high light reduction and activation of redox-modulated enzymes occurred, but then the stroma became relatively oxidized as a result of photoinhibition. The leaf contents of glucose 6-phosphate and fructose 6-phosphate increased following exposure to high light but subsequently decreased, suggesting that following photoinhibition sucrose synthesis exceeded the rate of carbon assimilation. The ATP content attained a constant value much higher than that in low light. During photoinhibition the glycerate 3-phosphate content greatly increased while ribulose-1,5-bisphosphate decreased. The fructose-1,6-bisphosphate and triose phosphate contents increased initially and then remained constant. During photoinhibition CO(2) assimilation was not limited by ribulose-1,5-bisphosphate carboxylase activity but rather by the regeneration of the substrate, ribulose-1,5-bisphosphate, related to a restriction on the supply of reducing equivalents.

Effect of high light intensities on oxygen evolution and the light activation of NADP-malate dehydrogenase in intact spinach chloroplasts

The factors limiting the photosynthetic carbon metabolism of intact spinach (Spinacia oleracea L.) chloroplasts after a high-light pretreatment have been studied. Photosynthetic CO2 fixation was decreased and became more sensitive to the inhibitory effect of the cyclic-electron-flow inhibitor, antimycin A. Depending on the extent of photoinhibition, changing the balance of linear to cyclic electron flow by adding oxaloacetate and antimycin A either did not relieve, or partially relieved the photoinhibitory effect. The decrease in CO2 fixation appeared to be the consequence of either a limitation by photosystem-II activity (in the case of moderate inhibition) or, at least partially an unfavourable balance between the linear and cyclic electron flows (in the case of strong inhibition). The light activation of NADP-malate dehydrogenase (EC 1.1.1.82) was decreased only in the presence of CO2, i.e. when there was strong competition for reducing power; otherwise, it was unaffected by photoinhibitory treatments, in accordance with its low energy requirement.

Modulation of NADP-Malate Dehydrogenase Activity in Maize Mesophyll Chloroplasts

When intact mesophyll chloroplasts of Zea mays var Kelvedon Glory were illuminated, activation of NADP-malate dehydrogenase occurred. Activity declined rapidly on darkening. Light activation of the enzyme was very much greater in the presence of pyruvate ( approximately 10- to 20-fold) than with the electron acceptors 3-phosphoglycerate or oxaloacetate present ( approximately 2-fold). Following preillumination in the presence of pyruvate, addition of 3-phosphoglycerate, oxaloacetate, or nitrite substantially diminished the activity of NADP-malate dehydrogenase. In these circumstances, with pyruvate and 3-phosphoglycerate present, activity could be restored by the addition of nigericin or dihydroxyacetone phosphate. Nigericin also restored activity with both oxaloacetate and pyruvate present. The effect of nitrite was more marked in the presence of low concentrations of DCMU.These observations are discussed in terms of the dependence of enzyme activity upon the redox state of ferredoxin and electron carriers; the redox state of the latter was estimated by analysis of the DCMU-induced relaxation kinetics of chlorophyll fluorescence quenching in the presence of different substrates.

Regulation of fructose-1,6-bisphosphatase activity in leaves

Fructose-1,6-bisphosphatase (EC 3.1.3.11) activity increased markedly (greater than 10-fold) upon illumination of wheat leaves. Darkening caused a relatively slow but complete reversal of light activation. The effects of O2 and CO2 concentration and light intensity on fructose-bisphosphatase activation were measured. In ratelimiting light, 2% O2 stimulated enzyme activity, whereas varying the CO2 concentration had little effect. In saturating light, lowering the oxygen tension had no effect, but CO2 at near-saturating concentrations for photosynthesis inhibited enzyme activity. Dark inactivation of the enzyme was completely prevented by incubation of leaves in N2, but was facilitated by O2, indicating that O2 is the major oxidant in darkened leaves. It is argued that while fructose bisphosphatase is redox-regulated in leaves, modulation of enzyme activity by this mechanism is unlikely to contribute to the regulation of CO2 fixation in leaves.